<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        /* 弹窗 (background) */

        .modal {
            display: none;
            /* 默认隐藏 */
            position: fixed;
            /* 固定定位 */
            z-index: 1;
            /* 设置在顶层 */
            left: 0;
            top: 0;
            width: 100%;
            height: 100%;
            overflow: auto;
            background-color: rgb(0, 0, 0);
            background-color: rgba(0, 0, 0, 0.4);
        }

        /* 弹窗内容 */

        .modal-content {
            background-color: #fefefe;
            margin: 15% auto;
            padding: 20px;
            border: 1px solid #888;
            width: 80%;
        }

        /* 关闭按钮 */

        .close {
            color: #aaa;
            float: right;
            font-size: 28px;
            font-weight: bold;
        }

        .close:hover,
        .close:focus {
            color: black;
            text-decoration: none;
            cursor: pointer;
        }
    </style>
</head>

<body>
    <!-- 打开弹窗按钮 -->
    <button id="myBtn">打开弹窗</button>

    <!-- 弹窗 -->
    <div id="myModal" class="modal">

        <!-- 弹窗内容 -->
    <div class="modal-content">
        <div class="modal-header">
          <span class="close">&times;</span>
          <h2>弹窗头部</h2>
        </div>
        <div class="modal-body">
          <p>弹窗文本...</p>
          <p>弹窗文本...</p>
        </div>
        <div class="modal-footer">
          <h3>弹窗底部</h3>
        </div>
      </div>

    </div>
</body>
<script>
    // 获取弹窗
    var modal = document.getElementById('myModal');

    // 打开弹窗的按钮对象
    var btn = document.getElementById("myBtn");

    // 获取 <span> 元素，用于关闭弹窗
    var span = document.querySelector('.close');

    // 点击按钮打开弹窗
    btn.onclick = function () {
        modal.style.display = "block";
    }

    // 点击 <span> (x), 关闭弹窗
    span.onclick = function () {
        modal.style.display = "none";
    }

    // 在用户点击其他地方时，关闭弹窗
    window.onclick = function (event) {
        if (event.target == modal) {
            modal.style.display = "none";
        }
    }
</script>

</html>